As to a high power output semiconductor laser device, how to improve the optical output is of particular importance, but today a method of increasing the optical output includes assembling a plurality of semiconductor lasers in stacked fashion.
FIG. 9 is a view showing the conventional high output semiconductor laser device described in, for instance, Japanese Published Patent Application No. Hei 6-90063.
This high output semiconductor laser device is made by first putting two semiconductor laser chips 108 one upon another and then assembling them in stacked fashion by soldering them together by means of a solder 108a. The semiconductor laser chip 108 is made by growing on an n.sup.+ type GaAs substrate 181 an n type Al.sub.x Ga.sub.l-x As cladding layer 182, an undoped GaAs active layer 183, a p type Al.sub.x Ga.sub.l-x As cladding layer 184, and a p.sup.+ type GaAs contact layer 185, successively, and forming a p side electrode 187 via an insulation layer 186 for current confinement on the contact layer 185, and an n side electrode 188 on the rear surface of the substrate 181.
When such a 2-chip laminated type semiconductor laser device is pulse driven under conditions of 50 ns in pulse width, 0.025% in pulse duty ratio and 25 amperes in input current, 30 W is obtained as the peak output power. The number of stages of semiconductor laser chips 108 may not necessarily be two as mentioned above, and it may as well be three or more and today even a semiconductor laser device laminating at maximum six chips and having a peak output of 100 W is on the market.
FIG. 10 is a perspective view showing a prior art surface light emission semiconductor laser which enables an optical output high in intensity and narrow in beam radiation angle, FIG. 11(a) a sectional view taken along line 11a--11a of FIG. 10 and FIG. 11(b) a partial sectional view taken along line 11b--11b of FIG. 10.
In the figures there are formed on an n type InP substrate 201 an n-type InGaAsP waveguide layer 202 having a composition producing a band gap energy corresponding to a wavelength of about 1.3 .mu.m, an undoped InGaAsP active layer 203 having a composition producing a band gap energy corresponding to a wavelength of about 1.55 .mu.m, a p type InP cladding layer 204, and a p type InGaAsP cap layer 205, laminated successively. These layers 202-205 can be grown by LPE (liquid phase epitaxy), MO-VPE (metal organic vapor phase epitaxy), MBE (molecular beam epitaxy) or the like. On the rear surface of the substrate 201 there is provided an n side electrode 206, and a p side electrode 207 is provided on the cap layer 205.
The resonator is constituted by the facets 208a and 208b formed by cleavage or etching and on the facets 208a and 208b there are formed a metal film 210 or dielectric film via an insulating film 209, resulting in high reflection facets. The metal film 210 surrounds the active layer 203 in a ring shape. A ring shaped diffraction grating 211 for optical output provided in the laser resonator enables emission of light perpendicular to the surface of the substrate. The diffraction grating has a secondary order with a period of about 5,000 angstroms (500 nm). The diameter of the ring shaped diffraction grating 211 is about 50 .mu.m-200 .mu.m. Excessive loss of light, other than by emitted light, is avoidable by forming the ring shaped diffraction grating 211 on the exposed low loss waveguide layer 202 after removing the active layer 203 and the like in a circle in the central portion of the laser resonator. Light emission by the ring shaped diffraction grating 211 is performed in the direction perpendicular to the diffraction grating formation surface but it is possible to obtain all optical output from above the diffraction grating formation surface by making the n side electrode 206 a reflection film or by providing a separate high reflection film thereunder.
The feature of this prior art device also resides in that it is possible to obtain the optical output from a plurality of laser resonators through one aperture by making the diffraction grating ring shaped and arranging a plurality of stripe configuration laser resonators radially. That is, the ring shaped diffraction grating 211 is provided at the center of the device, and four stripe configuration laser resonators 220, 221, 222 and 223 are arranged radially, traversing the ring shaped diffraction grating 211. The width of each laser resonator is 1-2 .mu.m for lateral mode control and its periphery is buried by, for example, a semi-insulating InP layer 212 as shown in FIG. 11(b), whereby leakage current is suppressed. Since in such a construction the output from each laser resonator is emitted upward from only the ring shaped diffraction grating 211, it is possible to obtain a high output in proportion to the number of laser resonators through a single aperture.
Since, as mentioned above, the high output semiconductor laser device shown in FIG. 9 has a plurality of semiconductor laser chips laminated by soldering, there were problems that a fault occurred in the light emission pattern due to insufficient lamination precision and that the laser chip was destroyed by thermal damage in the laser chip adhering step, resulting in difficulty in obtaining a high yield. Further, there was a problem of increased labor cost in the laser chip stacking process. In addition, this problem became more serious upon increasing the number of laser chips adhered together for still higher laser output.
The high output semiconductor laser device shown in FIG. 10 is structured such that light is taken out perpendicular to the substrate by means of a diffraction grating. Since in this case only the light of wavelength causing Bragg reflection in the diffraction grating is taken out, the optical output obtainable is only several percent of the optical output of the Fabry-Perot type laser device, resulting in difficulty in providing a high output semiconductor laser device required for a laser radar. Still other problems are complexity and increased cost due to formation of a diffraction grating with high precision.